Sprockets are an integral component of many heavy-duty equipment and machinery drive systems including those of conveyors, such as armored face conveyors and beam stage loaders used in underground mining of coal. By definition, such sprockets include a series of spaced teeth for engaging the sequential horizontal links of a chain being driven. Most often in this specific application, the sprocket accommodates dual, side-by-side chains. Each chain is captured between two contoured inside surfaces forming the teeth of the driving interface.
While such a rotary drive system has proven to be efficient and effective for this purpose, rapid wear of the teeth can be a serious problem. This wear primarily results from the harsh operating environment of mining equipment. Heavy loading, relatively high operating speeds and the presence of particulate matter, including coal fines, sand and small rock particles, trapped at the driving interface between the sprocket teeth and chain, causes the accelerated wear.
In order to facilitate repair/replacement of a worn or damaged sprocket, it has long been known to utilize a sprocket of split design. Such sprockets are typically split diametrically into two sections. This facilitates removal from the drive shaft, and replacement with a new sprocket. The split configuration of the sprocket eliminates the need to free one end of the drive shaft from the bearings for slipping the sprocket on and off over the end of the shaft. An example of such a split sprocket design is shown in, for example, the U.S. Pat. No. 4,037,713 to Soliman et al. Another known arrangement is where a split retainer assembly is used, such as shown in the U.S. Pat. No. 3,685,367 to Dawson. However, in this instance releasable stub shafts, or other arrangements must be made to remove the sprocket.
In these patents, the split sprocket or collar sections are retained on the drive shaft solely by means of cross fixing bolts mounted in the typical pattern transverse to the shaft axis. While such a fastening mechanism allows for proper and dependable operation for many light to medium duty applications, it leaves much to be desired insofar as providing the necessary strength to handle the torque generated radial thrust loads that may be anticipated to occur during certain heavy duty conveyor applications, or related rotary drive operations.
For example, the cross fixing bolts are in some instances simply not strong enough to continuously handle the torque/thrust loads required to transmit high horsepower, especially on some extended length armored face conveyors or beam stage loaders. Specifically, over time the bolts have a tendency to work loose or undergo metal stretch and/or fatigue. Either of these conditions can cause the sprocket and retainer assembly to slip on the drive shaft and eventually fail before being noticed by operating personnel. Not only must the sprocket then be replaced, but the failure also often causes the destruction of the driving key and/or damage to the drive shaft. The resulting necessary repairs to these structures significantly increases the downtime of the equipment. Of course, the replacement parts and the labor costs for repair are also increased. Thus, it should be appreciated that this is an area where innovation would be highly beneficial, and substantial money savings could be realized.
Other designs for retainer assemblies for holding together split rotary drive units are disclosed in U.S. Pat. Nos. 3,220,273 to Christian and 4,506,559 to Francke et al. In Christian, the sprocket is divided into two halves, with the dividing line extending in the plane of travel of the chain. A one-piece retaining ring is positioned between the two halves to hold the split sprocket sections together in the radial direction. A split retaining ring must be bolted to the hub supporting the sprocket halves to prevent axial displacement of the sprocket sections, and thus hold the entire unit together. While this design provides more reliable operation and better dissipation of the forces tending to pull the sprocket sections radially apart during machinery operation, it should be appreciated that the sprocket is weakened by having to be split into two halves. Also, the removal and replacement of the sprocket halves requires a tedious disassembly operation, including the removal of a large number of relatively small bolts. These same bolts must then be reinstalled during reassembly when the retaining ring is reattached to the hub of the sprocket. The installation of these relatively large number of bolts in the proper sequence, and then tightening to the proper torque setting, is a time consuming task. In addition, this retainer assembly also requires a shear pin to be installed. Dealing with these many, relatively small parts simply adds to the tedium and difficulty of replacement. Hence, repair and replacement is inconvenient, requiring significant labor expense and equipment downtime, negatively impacting productivity.
In Francke et al '599 patent, raised lateral segments are provided on split sections. These segments fit into cooperating grooves formed in disk-shaped, outer flanges that secure the assembly together. Once again, while this design may provide suitable force dissipation in light-to-medium duty pulley applications, it would not be suitable for use in heavy duty mining conveyor sprocket applications. The desired maximum strength of connection of the sections is clearly not achieved to allow full dissipation of anticipated, heavy duty forces, such as encountered during armored face conveyor operation. The segments fitting into the grooves incorporate an inherently weak shear coupling design. The presence of this teaching in a recent patent of using a shear coupling to connect a split rotary drive unit, simply reinforces the need for innovation and improvement in this area.